Kristoffer Ole Menzel

On the origin of dust vortices in complex plasmas under microgravity conditions

Under microgravity conditions, microparticles in a radio-frequency plasma form an extended dust cloud. In such clouds, self-excited large-scale vortices are observed. New experimental observations are reported, which exhibit a simple double vortex structure or a more complex quadrupole-like topology. Modeling the fields of the main acting forces, namely, the electric field force and the ion drag force, and calculating the curl of these forces reveal their non-conservative character and the possible driving mechanism of the vortices. It is shown that the curl of the ion drag force and of the electric field force has opposite sign and the combination could thus lead to the complex structures, also found in the observations.

The Structure of Self-Excited Dust-Density Waves Under Microgravity

Dusty plasmas under microgravity conditions are a great opportunity to observe dynamical processes in strongly coupled systems. For example, in such systems, self-excited dust-density waves can occur at low gas pressures in extended regions of the discharge. Recently, we have performed a series of measurements in a parallel-plate RF reactor during parabolic flights. It reveals that the waves can appear in two completely different states. One of them yields a high spatial and temporal coherence of the density fluctuations. This feature allows us to utilize scanning video microscopy to obtain information on the structure of the 3-D wave field. Under different experimental conditions, we also found that a wave field with multiple different wavelengths can arise in the dust volume. This results in defects in the wave pattern due to merging wavefronts. We determine their temporal evolution, which can be derived accurately from the phase information.

Experimental Investigation of Dust Density Waves and Plasma Glow

Dust density waves (DDWs) are compressional modes that are often excited by subsonic ion flows in dusty plasmas. Previous experiments relying on imaging of only the dust revealed that they can propagate parallel to the ion flow direction or at an oblique angle. An experiment was performed using microgravity conditions on parabolic flights with video imaging of both the dust and the plasma glow. Glow arises from electron-impact excitation of neutral gas atoms, and it serves as a signature of energetic electrons. Averaging over time, it was found that the presence of dust enhances the glow brightness everywhere in the plasma. Resolving the time variation, a spontaneously excited DDW was observed at 3.9 Hz. It was characterized not only by a compression of the dust number density but also by a modulation of the glow intensity. The correlation between the wave and the glow is analyzed by Fourier methods. We found an unexpected phase relation between the plasma glow and the DDW of 118o. A glow maximum is followed by a dust density maximum.

Stereoscopy of dust density waves under microgravity: Velocity distributions and phase-resolved single-particle analysis

Experiments on dust-density waves have been performed in dusty plasmas under the microgravity conditions of parabolic flights. Three-dimensional measurements of a dust density wave on a single particle level are presented. The dust particles have been tracked for many oscillation periods. A Hilbert analysis is applied to obtain trajectory parameters such as oscillation amplitude and three-dimensional velocity amplitude. While the transverse motion is found to be thermal, the velocity distribution in wave propagation direction can be explained by harmonic oscillations with added Gaussian (thermal) noise. Additionally, it is shown that the wave properties can be reconstructed by means of a pseudo-stroboscopic approach. Finally, the energy dissipation mechanism from the kinetic oscillation energy to thermal motion is discussed and presented using phase-resolved analysis.

Oscillation Amplitudes in 3-D Dust Density Waves in Dusty Plasmas Under Microgravity Conditions

Large dust clouds in dusty plasmas exhibiting self-excited dust density waves (DDWs) have been investigated in a microgravity environment. With the help of the tracer particle technique [1], 3-D trajectories of single dust particles within the dust cloud have been measured using a stereoscopic camera setup. With the availability of the full phase-space information, 3-D wave properties can be accessed. In this paper, the spatial variation of the oscillation amplitude of single particles participating in a DDW is presented. We find that the amplitude increases in the direction of wave propagation and is nearly homogenous in the plane perpendicular to that direction.

Three-Dimensional Force Field Measurements in the Void of Dusty Plasmas Under Microgravity Conditions

In dusty plasmas, when the gravitational force on the particles is compensated, usually an extended dust cloud with a central dust-free void emerges. Fast particles are injected into the void and followed along their trajectories in three dimensions. They act as probes to measure the forces forming the void. An image illustrating this three-dimensional resolved force field inside the void is presented.

Interactions Between Dust-Density Waves and Plasma Glow

In complex plasmas, self-excited dust-density waves (DDWs) are observed below a critical gas pressure. Under microgravity conditions, the dust cloud fills the entire discharge volume so that extended wave fields can be generated. Here, we study the relationship between plasma glow and DDWs. Analyzing the dust density and plasma glow by means of video microscopy reveals self-organized oscillations of the entire plasma. In the region of strongly modulated DDWs, also waves occur in the plasma glow, having wavelengths and frequencies comparable to those of the DDWs.

Modeling Dust-Density Wave Fields as a System of Coupled van der Pol Oscillators

Dust-density wave fields in radio-frequency discharges under microgravity conditions exhibit complex spatiotemporal patterns. One of the most remarkable features, which reflects this behavior, is the occurrence of so-called frequency clusters, i.e., a stepwise spatial variation of the frequency inside the wave field. These frequencies were found to be incommensurable to each other. Such topological phenomena can be emulated by networks of locally coupled van der Pol oscillators. However, recent investigations showed that, in some situations, clusters with commensurable frequencies are preferred. The previously suggested model cannot explain such a preference. Hence, in this contribution, an extension of the existing model is proposed that, in addition to the local coupling, includes a global coupling mechanism.

Anode Surface Temperature Determination in High-Current Vacuum Arcs by Different Methods

The electrode surface temperatures of Cu–Cr butt electrodes exposed to vacuum arcs with sinusoidal currents of 10–20 kA and under external axial magnetic field were determined. Different experimental techniques were applied that can be distinguished by the used spectral wavelength range, their temporal, and spatial resolution. Near infrared spectroscopy was carried out by means of a fiber optic spectrometer working in the wavelength range from 900 to 1670 nm with a temporal resolution of 1–2 ms. Electrode surface temperatures after current zero were obtained from the relative shape of the spectrum using the Planck curve fitting procedure. Furthermore, electrode emissivities were derived after performing absolute calibration of the spectra. Pyrometric measurements were performed in the spectral range around 2

Wave Crest Reconstruction of a Dust Density Wave Using Single Particle Trajectories

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